<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="number" id="i1">
<input type="number" id="i2">
<input type="button" value="加" onclick="f1()">
<input type="button" value="减" onclick="f2()">
<input type="button" value="乘" onclick="f3()">
<input type="button" value="除" onclick="f4()">
<h3>
    结果:<span></span>
</h3>
<script>
    let a = document.querySelector("#i1");
    let b = document.querySelector("#i2");
    let c = document.querySelector("span");

    function f1() {
        if (isNaN(a.value) || isNaN(b.value)) {
            c.innerText = "请输入数字!";
            return;
        }
        c.innerText = a.value * 1 + b.value * 1;
    }

    function f2() {
        if (isNaN(a.value) || isNaN(b.value)) {
            c.innerText = "请输入数字!";
            return;
        }
        c.innerText = a.value - b.value;
    }

    function f3() {
        if (isNaN(a.value) || isNaN(b.value)) {
            c.innerText = "请输入数字!";
            return;
        }
        c.innerText = a.value * b.value;
    }

    function f4() {
        if (isNaN(a.value) || isNaN(b.value)) {
            c.innerText = "请输入数字!";
            return;
        }
        c.innerText = a.value / b.value;
    }
</script>
</body>
</html>